The invention relates to stabilized compositions of O- or N-vinyl compounds, to a process for stabilizing O- or N-vinyl compounds, and to the use of the stabilizers.
Vinyl compounds where the vinyl group or the vinyl groups have bonding via oxygen or nitrogen to the main skeleton of the compound, known as O- or N-vinyl compounds, have a tendency toward spontaneous and strongly exothermic cationic polymerization, in particular in the presence of acid. In the case of vinyl ethers, furthermore, creeping polymerization to give polyvinyl ethers is observed, and vinylamines/amides are similarly found to give polyvinylamines/amides. Even storage under inert conditions does not inhibit the polymerization.
O-Vinyl ethers, which are of particular commercial importance, are used as monomers in the production of a wide variety of homo- or copolymerized products, and serve as reactive diluents in surface coatings. In this application sector it is essential to inhibit premature polymerization. When O-vinyl ethers are stored under inert conditions, the presence of polyvinyl ether oligomers can be detected even after a few weeks with the aid of gel permeation chromatography (GPC). The monomer content determined by gas chromatography (GC) decreases correspondingly. The formation of toxic acetaldehyde also indicates the decomposition of the O-vinyl ether. Without addition of a stabilizer, even after a few weeks the acetaldehyde content is sometimes 800 ppm or above. If O-vinyl ethers also contain other functional groups, such as hydroxyl, spontaneous intra- or intermolecular acetalization can occur.
Stabilizers which have been known for a long time are alkali metal hydroxides, specifically KOH (Ullmann""s Encyclopedia of Industrial Chemistry, volume 27, 5th edition, pp. 435-441, specifically p. 440, VCH-Verlagsgesellschaft mbH, Weinheim). However, these stabilizers, such as KOH, are not soluble in every vinyl ether, and this means that relatively large amounts of hydroxides have to be added and settle at the base of the vessels. The use of relatively large amounts of hydroxides is problematic, since the hydroxides comprise considerable amounts of water and also sometimes liberate water by deprotonating alcohols. This water brings about the decomposition of vinyl ethers into acetaldehyde and alcohol. Acetaldehyde is slightly toxic and causes odor-impairment of the product. It is also known that the use of KOH brings about discoloration of the product. This reduces the scope of application and reduces the maximum storage time of vinyl ethers.
U.S. Pat. No. 5,691,462 discloses that the alkali metal salts of an unsubstituted C1-C14 carboxylic acid can be used as stabilizers for vinyl ethers. Examples mentioned are sodium acetate, potassium acetate, sodium carbonate and potassium carbonate. Although the vinyl ether compositions stabilized with these substances are more colorfast than KOH-stabilized compositions, the suppression of acetaldehyde formation is unsatisfactory.
DE-A-1 286 015 stabilizes aromatic vinyl compounds, such as styrene, with respect to polymerization by adding, as stabilizers, hydroxyammonium acetates of the formula CH3COO[NH(OH)R1R2], where each of R1 and R2 is alkyl having from 1 to 6 carbon atoms.
DE-A 196 09 312 stabilizes monomer compositions with respect to polymerization by adding N-oxyl compounds of a secondary amine which has no hydrogen atoms on the xcex1-carbon atoms, the monomers having certain vinyl groups.
It is an object of the present invention to provide stabilized compositions of vinyl compounds which firstly have good colorfastness and secondly have a low decomposition rate. A further object of the invention is to provide substances which can be used as stabilizing additives to vinyl compounds and which give the resultant compositions the properties mentioned. The additives should be low-cost, have satisfactory solubility, and also be capable of removal from the vinyl compounds.
We have found that this object is achieved by means of a composition of vinyl compounds, comprising
a) at least one polymerizable compound which has at least one O-vinyl group or one N-vinyl group, and
b) at least one ammonium salt of the formula (I) 
where R is NH2, Oxe2x88x92 NH4+, C1-C18-alkyl, C6-C18-cycloalkyl, or C7-C18-alkylcycloalkyl.
Ammonium salts of the formula (I) can be prepared in relatively anhydrous form and are low-cost. Addition of these ammonium salts to the polymerizable compounds of component a) has been found to suppress premature polymerization of these compounds, while the change in color of the resultant composition of the invention is markedly smaller than with addition of the known compounds. It has moreover been found that the inventive ammonium salts of the formula (I) dissolve or disperse relatively readily in the polymerizable compounds of component a), and are nevertheless readily removable. The resultant compositions of the invention are easy to pump.
For the purposes of the present invention, a polymerizable compound which has at least one O-vinyl group or one N-vinyl group is a heterosubstituted vinyl monomer bearing oxygen or nitrogen as heteroatom at the vinyl group. Examples of heterosubstituted vinyl monomers which may be used are vinyl carboxylates, such as vinyl acetate, vinyl propionate or vinyl butyrate, vinyl ethers, such as methyl vinyl ether, ethyl vinyl ether or butyl vinyl ether, triethylene glycol vinyl ether, hydroxyalkyl vinyl ethers, such as hydroxybutyl vinyl ether, and cycloalkyl vinyl ethers, such as cyclohexyl vinyl ether, and also vinylcarbazoles, vinylpyrrolidones, vinylphthalimides, vinylcaprolactams, vinylimidazoles and vinylformamide.
Preferred ammonium salts are ammonium carbonamate, ammonium carbonate and the ammonium salts of methanoic, ethanoic, propanoic, isopropanoic, butanoic, isobutanoic, tert-butanoic, pentanoic, isopentanoic, hexanoic or cyclohexanoic acids, and mixtures of these.
Particularly preferred ammonium salts are ammonium carbonate ((NH4)2CO3) and ammonium carbamate (NH4NH2CO2).
In another embodiment of the composition of the invention, at least one polymerizable compound of component a) is covered by the formula (II)
H2xe2x95x90CHxe2x80x94Xxe2x80x94R1xe2x80x83xe2x80x83(II),
where
X is O or NR2,
R1 is 
xe2x80x83or R3,
R2 is hydrogen or C1-C4-alkyl or, together with R3, is a C3, C4, or C5 alkylene bridge or alkenylene bridge, in which up to two non-adjacent CH2 groups may have been replaced by NH, N(C1-C4-alkyl), N(C6-C10-aryl) or oxygen, and up to two non-adjacent CH groups may have been replaced by N, and
R3 is hydrogen or a singly or multiply hydroxyl-substituted or vinyloxy-substituted or unsubstituted C1-C16-alkyl-, C6-C16-cycloalkyl- or C1-C4-alkyl-C6-C12-cycloalkyl group or, together with R2, is a C3, C4 or C5 alkylene bridge or alkenylene bridge, in which up to two non-adjacent CH2 groups may have been replaced by NH, N(C1-C4-alkyl), N(C6-C10-aryl) or oxygen, and up to two non-adjacent CH groups may have been replaced by N.
X in the polymerizable compounds of the formula (II) present in the composition of the invention is preferably oxygen. Vinyl ethers where R1 is C1-C4-alkyl, i.e. methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl or tert-butyl, are preferred polymerizable compounds. Preference is also given to vinyl ethers where R1 is C1-C4-hydroxyalkyl or C1-C4-alkyl with one vinyloxy substituent.
Other embodiments of the composition of the invention are those which comprise, as polymerizable compound, triethylene glycol divinyl ether (DVE), 4-hydroxybutyl vinyl ether, and/or cyclohexyl vinyl ether. If X is NR2, R1 is preferably COxe2x80x94R3.
Radicals R3 which may be used, besides hydrogen and the C1-C4-alkyl groups mentioned, are radicals which together with NR2 form a saturated or unsaturated 5- to 7-membered ring. Examples of ring systems of this type are those of the formula (III) 
In another embodiment of the composition of the invention, Xxe2x80x94R1 is NH2COCH3, N-imidazolyl, N-pyrrolidinonyl or N-caprolactamyl.
The invention further provides the use of an ammonium salt of the formula (I) as additive to polymerizable compounds which have at least one O-vinyl group or one N-vinyl group.
Another embodiment of the invention is the use of ammonium carbonate ((NH4)2CO3) or of ammonium carbamate (NH4NH2CO2) as additive to polymerizable compounds which have at least one O-vinyl group or one N-vinyl group.
The invention further provides a process for inhibiting premature polymerization of compositions which comprise polymerizable compounds which have at least one O-vinyl group or one N-vinyl group, which encompasses adding to the compositions an ammonium salt of the formula (I).
In another embodiment of the process of the invention, the ammonium salt used is ammonium carbonate ((NH4)2CO3) or ammonium carbamate (NH4NH2CO2).